In JP Patent Kokai JP-A-5-171932, there is disclosed a conventional exhaust manifold made up of an upper member and a lower member separated from each other at a plane passing substantially through the center axis of a collecting tube and four branched tubes connected to the collecting tube, with the outer two branch tubes being bent substantially at right angles to the collecting tube. On the outer periphery of the upper member and the lower member are formed joining flanges of the same width. At the connecting portions of the collecting tube and the branch tubes, intermediate between the branch tubes, there is mounted a partition for adjusting the exhaust gas stream and reducing the noise of interference of the exhaust gas streams flowing via the respective branch tubes. The exhaust manifold is assembled by connecting the upper and lower members, each formed from a sole metal plate by pressing or sheeting, by joining flange portions formed at the outer periphery of the upper and lower members, and welding the outer peripheral area in its entirety.
However, by connecting plural branch tubes to the sole collecting tube, there is produced a difference in length of the branch tubes, such that a few of the branch tubes become bent. Above all, in the case of an exhaust manifold for an internal combustion engine for a vehicle, the degree of bending of the branch pipes is further increased because of limitations on the external structure of the exhaust manifold and due to necessity of space saving.
On the other hand, when connecting such exhaust manifold to the exhaust side of the cylinder of an internal combustion engine, the hot exhaust gas flows intermittently into the exhaust manifold, so that the exhaust manifold is subjected to thermal shock or stress due to repeated heating and cooling. Thus the non-linear portion, especially the bent portion of the branch tube, is susceptible to crevices, as a result of which the exhaust manifold is lowered in durability.
In JP UM kokai publication 3-25815, there is proposed an exhaust manifold communicating with the exhaust port of the multi-cylinder engine and having plural branch tubes, in which bellow-shaped thermal deformation absorbing portions are provided at an area of the branch tubes undergoing larger thermal deformation.
However, the provision of such bellows in the branch tubes leads to complex shape and difficulties in machining. Above all, if such exhaust manifold is fabricated by joining an upper half member and a lower half member, difficulties are met in matching the joining surfaces of the upper and lower members with high precision, with the result that machining costs are increased.
In JP Patent Kokai JP-A-5-171932, there is disclosed an exhaust manifold in which a partition wall is provided in the collecting tube with a view to preventing larger interference noises or impact noises from being produced by sudden impact and interference of the effluent exhaust gas flows from the branch tubes, thereby preventing an engine output of the exhaust engine from being lowered.
However, the provision of such partition wall produces the impact of the effluent exhaust gas flows from the branch tubes onto the partition wall or the sudden confluence of the effluent exhaust gas flows from the respective branch tubes at a high pressure at a portion where the partition wall is depleted, thereby still producing the interference noises of the exhaust gas.